The invention generally relates to electric machines, and more specifically, to permanent magnet electric generators and to voltage and current control systems for such generators.
Electric generators are well known in a variety of applications. Small generators are commonly used for automobile electrical systems, portable machines, and emergency systems. Similarly, larger generators provide power to large equipment, structures, and communities. Because of the wide variety of applications and needs, generators continue to develop, providing greater and higher quality power for a wide range of applications.
Regardless of its size or application, a generator typically comprises a rotor mounted on a rotating shaft and disposed concentrically relative to a stationary stator. Alternatively, a stationary rotor may be positioned concentrically within a rotating stator. An external energy source, such as a motor or turbine, commonly drives the rotating element. Both the stator and the rotor have a series of poles. Either the rotor or the stator generates a magnetic field, which interacts with windings on the poles of the other structure. As the magnetic field intercepts the windings, an electrical current is generated, which is provided to a suitable load. The induced current is typically applied to a bridge rectifier, sometimes regulated, and provided as an output. While typically not found in portable units, an AC output can be provided by applying the DC signal to an inverter.
Permanent magnet generators, as their name suggests, use permanent magnets to generate the requisite magnetic field. Permanent magnet generators tend to be much lighter and smaller than traditional wound field generators. The power supplied by a permanent magnet generator is difficult to regulate or control. First, the voltage supplied by the generator varies significantly according to the speed of the rotor. In many applications, changes in the rotor speed are common due to, for example, engine speed variations in an automobile, or changes in the load characteristics. In addition, the voltage of a permanent magnet generator varies inversely with the current delivered, i.e. as the current increases, the voltage drops. Such variations in the voltage are generally unacceptable for conventional loads, and must be strictly controlled.
This is particularly true in automotive applications. For example, an automotive engine typically idles at speeds on the order of 600 RPM. However, at highway speeds, the engine often runs at speeds an order of magnitude higher, e.g., 6,000 RPM. Accordingly, if a conventional permanent magnet alternator is required to provide operating voltage (e.g. 12 volts) while at idle speeds, it will provide multiples of the operating voltage, e.g., ten (10) times that voltage, at full engine RPM, e.g., 120 volts. Where the voltage at idle is 120 V, e.g. for electric drive air conditioning, the voltage at full engine RPM would be, e.g., 1200 volts. Such voltage levels are difficult and, indeed, dangerous to handle. In addition, such extreme variations in the voltage and current may require more expensive components; components rated for the high voltages and currents produced at high engine RPM (e.g., highway speeds) are considerably more expensive, than components rated for more moderate voltages.
Consequently, automobiles typically use claw-pole type alternators, notwithstanding the fact that a claw-pole type generator for a given power output is significantly larger and heaver than a corresponding permanent magnet alternator, and claw-pole type alternators are subject to size constraints that make such alternators difficult to use in high output applications, e.g., 5 kw, for powering air conditioning or refrigeration. In addition, claw-type generators are also disadvantageous in that voltage regulation is by modulating the rotating field. Such modulation effects all of the windings. Accordingly, voltage regulation and control of individual windings is impractical.
Similar problems arise with respect to the use of permanent magnet generators in other applications. It would be desirable to employ permanent magnet generators in electric welders. However, electric welders typically require a particular current to voltage relationship. For example, arc welders require an inverse slope of current to voltage, whereas metal inert gas (MIG) welders (wire feed welders) require a constant voltage and variable current and tungsten inert gas (TIG) welders require a constant current and variable voltage. Since permanent magnet generator's outputs are dependant upon motor speed, they are typically not suitable for electric welder applications. This is particularly true with respect to multipurpose welders that provide a plurality of electrical welding types.